This invention relates to the recycling and recovery of effluent gas generated during the production of aluminum from aluminum chloride. More particularly, it relates to a method of and apparatus for cleaning filter bags contaminated with aluminum chloride during the recycling and recovery of effluent gas.
In the production of aluminum by electrolysis of aluminum chloride in an electrolyte, one of the many problems that had to be overcome involved the handling and disposition of effluent gas from the electrolytic reduction cell. One method for recycling and recovering effluent gas is described in U.S. Pat. No. 3,904,494, issued Sept. 9, 1975, which generally relates to the selective condensation and separation of aluminum chloride and mixtures therewith resulting in a production of chlorine gas of relatively high purity and effectively free of condensable impurities therein. A demister is described for removing condensed liquid in droplet form on a mat, such as one made of quartz, glass, cloth bags or the like. The condensed liquid may include various alkali and/or alkaline earth metal halides in combination with aluminum chloride values along with small amounts of aluminum chloride. Additionally, residual effluent gas is further purified by removal of finer-sized solid contaminants, such as aluminum chloride, by passage of the gas through a permeable filter media, preferably a filter of the dry or fabric type.
Periodically, the mat or cloth bags of the demister and the dry or fabric type filter of the permeable filter become covered with solid contaminants and must be removed from the system to be replaced with new filtering materials. The disposal of the filter materials which are contaminated with aluminum chloride is the problem to which the present invention relates.
Generally, the demister and permeable filter include filtering means, such as units which utilize "tube sheets" and "bag cages", respectively, over which filtering material is positioned. Such filtering materials, for example cloth filter bags, during cleaning emit noxious gases and aluminum chloride thereon will chemically burn flesh if not properly handled and disposed of. Such proper disposal is in accord with the desire to provide a safer working environment. The referred to "tube sheets" and "bag cages" are further described hereinbelow.
In the past stripping of the filter bags was a very undesirable job. Sometimes water was sprayed on the bags while still on the tube sheets and bag cages to neutralize the aluminum chloride (AlCl.sub.3), but large quantities of fume, including Al.sub.2 O.sub.3, HCl and steam, were generated. Additionally, the bags may have been stripped from the tube sheets and bag cages while they were still contaminated with aluminum chloride.
Though hydrolysis of aluminum chloride is known in the art, and though it is common practice to wash aluminum chloride with water, there still exists a need to dispose of aluminum chloride contaminated filter bags in a safe, controlled manner without polluting the environment and without undue risk to operators.